There are several examples of injectable bone void fillers in the literature. WO 95/21634 discloses a biomaterial for the resorption substitution of bony tissue. The composition is injectable and comprises calcium phosphate particles in a liquid phase comprising carboxymethylcellulose. U.S. Pat. No. 6,287,341 details a method for repairing an osseous defect wherein two calcium phosphates are mixed with a buffer to provide a paste or putty which is applied to the defect. The putty hardens in the defect due to a chemical reaction. WO 00/07639 discloses a calcium cement for injection into osseous defects. The cement is based on mono basic calcium phosphate monohydrate and beta tricalcium phosphate and may further comprise a biopolymer. Following injection the calcium phosphate cement requires setting. US-patent 2004048947 details an injectable composition for a bone mineral substitute material with the capability of being hardened in a body fluid in vivo, which hardens during the surgery. US-patent 2004101960 details an injectable bone substitute material comprising a mix of living cells within a composition which comprises a soft matrix or a composition which comprises a setting material. The soft materials listed in this patent include collagen gels, gelatin, alginates, agarose, polysaccharides, hydrogels and viscous polymers. It is also mentioned that it is possible to employ commercial fibrin glues such as TissuCol (Baxter) or Beriplast (Aventis) but they are not preferred. Recently there have been a number of injectable bone void fillers that have received 510(k). Of these, Jax-TCP (Smith & Nephew) and Tricos T (Baxter) deliver granules of calcium phosphates in a bio-gel which are applied as a putty/paste.
The current practice is to fill bone voids with either a bone graft (auto or allograft), bone graft substitutes, a bone cement such as polymethylmethacrylate (PMMA) or injectable calcium salt void fillers. Autografts are the ‘gold standard’ choice for this application but there are issues with donor tissue limitations, trauma, infection and morbidity. There are a number of additional problems that face allografts, including the risk of disease transmission and immunogenicity. Both auto- and allografts display loss of biological and mechanical properties due to secondary remodeling. It is these limitations that have prompted interest in alternative materials to bone grafts (Parikh S. N., 2002, J. Postgrad. Med. 48:142-148).
PMMA is a non-resorbable polymeric material. During its polymerization unreacted monomer, catalyst and low molecular weight oligomers become entrapped in the polymer. These chemicals have the potential to leach out of the material resulting in localized cytotoxic and immunological responses. PMMA polymerization has a high exothermicity that can potentially cause heat necrosis. This exothermicity also limits the ability of PMMA to incorporate any pharmacological or chemotherapeutic agents. PMMA leakage from a defect can result in very serious complications including compression of adjacent structures (requiring further surgery) and/or embolism.
As indicated above, there are a number of calcium salt based “injectable void fillers” in the prior art. However moldable pastes also come under this heading. Putties and pastes require surgical placement of a defect. In practice this requires the defect to be surgically revealed. Unfortunately the larger the defect the larger the surgical wound site (US-patent 2005136038). Another major complication with calcium salts is their requirement to for setting in vivo. This is usually achieved by chemical reaction. Thus any biologics and pharmaceutics incorporated in the filler such as cells and pharmacological agents can potentially be damaged. Furthermore, if the filler is too “fluid” it can leak out of the defect into adjacent spaces leading to compression of structures. Leakage from defects proximal to joints can potentially impair the joints function.
Requirements for a calcium salt composition intended for delivery via the percutaneous route have previously been detailed in WO 95/21634. These include that the material should be sterilizable, must be non-toxic in vitro, the rheology must be such that it permits injection, it must be easy to use and it must have a strong mineralization front.
Thus, a strong need exists for new injectable bone void fillers which can be sterilized, show a low potential toxicity and a low tendency for leakage, are biodegradable, have a rheology that permits injection and are easy to use.
Therefore, it is an object of the present invention to provide new injectable void bone fillers for injection into osseous defects or voids resulting, for example, from osteoporosis, surgery, bone cysts, tumor removal or traumatic bone injury.